Flexible phase change material composite for thermal management systems

ABSTRACT

A thermal management composite, comprising a phase change material within a carbon or graphite matrix. The matrix is coated with a polymer coating to improve flexibility. The matrix can be a molded carbon or graphite material or a carbon or graphite cloth.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/105,768, filed 17 Jun. 2016, now U.S. patent Ser. No. 10/005,941,which is a National Phase Application of PCT/US2014/070740, filed on 17Dec. 2014, which claims the benefit of each of: U.S. ProvisionalApplication 61/917,142, filed on 17 Dec. 2013; and U.S. ProvisionalApplication 61/921,728, field on 30 Dec. 2013. The co-pending parentapplication is hereby incorporated by reference herein in its entiretyand is made a part hereof, including but not limited to those portionswhich specifically appear hereinafter.

BACKGROUND OF THE INVENTION Field of the Invention

This invention is directed to phase change material/graphite matrixcomposites that are widely used in thermal management battery systems,and more particularly to a phase change material/graphite matrixcomposite material that is compressible and flexible, and that canwithstand mechanical stresses without breaking apart or losing thermalcontact with cells

Discussion of Related Art

Phase change materials (PCM) are widely used in thermal managementsystems such as those used in lithium ion (Li-ion) batteries. Al-Hallajet al. disclose using PCM in thermal management of battery system inU.S. Pat. Nos. 6,468,689 and 6,942,944, herein incorporated byreference. PCM is usually impregnated inside pores of graphite matrixsurrounding Li-ion cells. The heat released by Li-ion cells is conductedaway by the graphite matrix to the PCM. The PCM absorbs large amount ofheat close to its phase transition temperature thereby stabilizing andcontrolling battery temperature to safe operating limits.

The PCM containing graphite matrix mentioned in the above Patents has afew drawbacks and limitations because of material composition of thegraphite matrix. Graphite matrix generally has low electricalresistivity which is not desirable because it can cause electricalshort-circuit between battery cells. Graphite matrix composite alsogenerally has poor mechanical characteristics such as has low mechanicalyield strength, brittleness, being inflexible, and being anon-compressible material. These mechanical characteristics can beimportant for Li-ion battery applications, such as for prismatic andpouch cells usage that exert mechanical stress on the graphite matrixcomposite because of their continuous expansion during charging anddischarging cycles. Prismatic cells (or pouch cells) are being widelyused in many high power and high energy density applications becausethey have high energy and power density, but they usually expand andcontract during charging and discharging cycles. Thus, there is acontinuing need for improvement in the graphite matrices, in boththermal properties of the composite, such as thermal conductivity andlatent heat, and for the mechanical properties, such as materialstrength, flexibility and resiliency.

SUMMARY OF THE INVENTION

An object of the invention is to improve flexibility and/orcompressibility in PCM composites. The object of the invention can beattained, at least in part, through a thermal management compositeincluding a carbon or graphite matrix and each of a phase changematerial and a polymer material on and/or within the matrix. The matrixcan be embodied, for example, as a molded composite or a woven and/ornonwoven fibrous (e.g., cloth) composite. The phase change material isdesirably dispersed within or throughout the matrix, such as disposed invoids between fibers or bundles of fibers of the matrix. A polymermaterial can be dispersed throughout the matrix as well and/or desirablyapplied as a coating to the molded or cloth matrix.

The invention further includes a method of making a thermal managementcomposite of this invention. The method includes impregnating a carbonor graphite matrix with a phase change material, either fully dispersedor localized, and then coating at least one side, and desirably allsides, of the impregnated matrix with a polymer coating. The coating canoptionally include additives, such as carbon or graphite powders orfibers. Dip coating and/or spray coating can be used to apply the phasechange material and/or the polymer coating. Embodiments of the methodfurther include mixing the phase change material with a polymer materialand a carbon or graphite material to obtain a matrix mixture, moldingthe mixture into a molded matrix, and coating at least one side of themolded matrix.

Other objects and advantages will be apparent to those skilled in theart from the following detailed description taken in conjunction withthe appended claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 representatively illustrates the effect of mechanical stress on ahigh PCM loaded composite without a protective coating of thisinvention.

FIG. 2 representatively illustrates the effect of mechanical stress on ahigh PCM loaded composite with a protective coating according to oneembodiment of this invention.

FIG. 3 illustrates a method of manufacturing a PCM composite materialaccording to one embodiment of this invention.

FIG. 4 shows images of carbon cloth, with FIG. 4A being woven carboncloth, FIG. 4B being knitted carbon cloth at a microscopic resolution,and FIG. 4C being woven carbon cloth at a microscopic resolution

FIG. 5 illustrates an elastomer coated flexible PCM composite accordingto one embodiment of this invention.

FIG. 6 shows the flexibility and relaxation of the elastomer coatedflexible PCM composite of FIG. 5

FIG. 7 graphically illustrates compositions of PCM composite materialand elastomer coatings according to embodiments of this invention.

FIG. 8 shows differential scanning calorimetry (DSC) results comparingenthalpy of melting for a control sample without elastomer and compositematerials with elastomer according to embodiments of this invention.

FIG. 9 summarizes mechanical compression testing of a control PCMcomposite (graphite matrix: 20%, PCM: 80%) and a flexible PCM compositesmade according to embodiments this invention.

FIG. 10 summarizes high temperature soaking test results for PCMimpregnated carbon cloth samples.

FIG. 11 shows DSC results of carbon cloth/PCM composite made accordingto embodiments of this invention.

FIG. 12 shows a coated PCM impregnated carbon cloth according to oneembodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention is in response to the above mentioned mechanical andthermal limitations and shortcomings of phase change material (PCM)composites and provides enhanced properties for carbon/graphitematrices, such as providing higher thermal conductivity and alsoimparting higher mechanical strength and structural integrity to supportthe PCM composite, which results in having good flexibility andcompressibility to withstand mechanical stresses.

The invention includes modifying a material composition of graphitecomposite by adding/mixing a polymer material within the compositematrix and/or including an outer coating containing a polymer material.The base composite contains PCM, graphite, and, optionally, a polymermaterial binder as primary ingredients. The outer coating contains apolymer material as a primary ingredient, along with any optional anddesirable additives, such as graphite powder and/or fibers. Together, afinal composite including the base composite material and outer coatingmaterial enhances mechanical properties, electrical resistivity, and/orreduces PCM leakage.

The invention provides a PCM composite that is compressible and flexibleand that can withstand higher mechanical stresses without breaking apartor losing thermal contact with cell. The invention includes a PCMcomposite that can be made in thin layers and/or wrapped or wound aroundprismatic cells. The invention also includes a PCM composite materialthat can be thermally bonded to improve thermal contact.

Flexibility and compressibility of the PCM composite of this inventionfor battery thermal management applications addresses current PCMcomposite mechanical strength limitations. The PCM composite willprovide new market applications specifically in Li-ion pouch cells thatrequire use of thin PCM composite slabs. PCM composite slabs accordingto embodiments of this invention can also be used in prismatic cellindustrial applications such as those in airplanes, automotive, andships.

The invention includes a PCM composite including a PCM, thermalconductive fillers, and a polymer material in various weight proportionsto achieve high latent heat, high thermal conductivity and goodmechanical strength, flexibility, resilience, compressibility. The PCMcomposite can have latent heat of >100 J/g for thermal managementapplications such as in battery, heat storage and release systems. ThePCM composite can also or additionally have high thermal conductivityfor thermal management applications such as in battery, heat storage andrelease systems. The PCM composite can have high electrical resistivity,such as >100 Ω·m, to reduce or eliminate electrical short betweenbattery cells in battery applications. The PCM composite can have highmechanical flexibility, resiliency, compressibility, and/or compressivestrength, to withstand mechanical stresses.

The invention includes a phase change material, which has predefinedmelting temperatures for different applications, such as optimum batteryoperations energy storage systems. The percentage weight of the PCM isbetween 30 to 80% in the composite material. The PCM can include morethan one type of phase change material with different meltingtemperatures in order to regulate battery temperature at differenttemperatures such as cold temperature, normal temperature, hightemperature for thermal runaway prevention, etc. In one embodiment ofthis invention, a PCM composite is sandwiched between cells to absorbheat released by cells and also to release some heat to cells. Theinvention also includes a PCM matrix configurable with holes or slots toinsert cells into a composite matrix. The PCM material of this inventionincludes all forms of phase change material, such as paraffin wax(widely used), vegetable based wax, etc., with different phasetransition temperatures. The PCM material can also be polymerencapsulated according to known methods.

The PCM can be used with any suitable matrix material. Exemplary matrixmaterials include carbon or graphite materials. In embodiments of thisinvention the matrix can be molded into any suitable form. In anotherembodiment the matrix is a carbon cloth. The PCM can be applied toand/or within the matrix in any suitable way, such as by dip or spraycoating. In embodiments of this invention, the phase change material isdispersed throughout the molded composite. A polymer material, such aspolymers described for the coatings herein, can also be dispersedthrough the matrix as a binder material.

FIG. 1 shows a schematic of a representative molded composite containingvery high loading of PCM (>40%) and elastomer binder and graphite. ThePCM composite cracks under fairly low mechanical stress because thepolymer binder (an elastomer preferably) cannot resist the stressbecause it is present in low loadings in comparison to PCM.

Embodiments of this invention include a polymer material or polymercomposite coating on an outside of the PCM containing matrix thatabsorbs mechanical stress, imparts mechanical flexibility, resiliency,compressibility to the PCM composite and also prevents the PCM compositefrom breaking under mechanical stress. FIG. 2 shows a schematic of a PCMcomposite containing high loading of PCM (for high latent heat),graphite (for high thermal conductivity) and polymer (like silicone,EPDM, etc. for flexibility, resilience, compressibility). This matrix iscoated with a polymer material and graphite mixture on the outside. Thepolymer coating can include thermal conductivity enhancing fillers suchas graphite powder, graphite fibers, milled fibers, carbon black, carbonnanotubes, metal oxides, metal powder, and/or polymer material toenhance electrical resistivity, mechanical strength, flexibility,resiliency, and/or compressibility. The polymer composite coating caninclude thermal conductivity enhancing fillers in a weight ratio of, forexample, 5 to 50%. The coating composition preferably, but notnecessarily, has the same polymer and graphite as in the base PCMcomposite to which coating is applied. The optional inclusion of thegraphite can be used to tailor coating characteristics. The amount ofgraphite and polymer weight ratio in the coating composition can bedetermined by how much thermal conductivity, electrical resistivity,flexibility, resilience and/or compressibility is required. For example,if high thermal conductivity is required, more graphite will be used incoating composition; if high electrical resistivity and/or flexibility,resilience, compressibility are desired, more polymer will be used incoating composition.

One exemplary polymer material is an elastomer compound that can beadded in varying weight fractions to achieve certain degree ofmechanical strength, flexibility, resiliency and compressibility. Manyelastomeric compounds exist today and are widely used in rubberindustries. Exemplary elastomeric polymers include PDMS silicone,polyurethane, EPDM rubber, EPM, polystyrene, polyisoprene, polyacrylate,SBS rubber, neoprene rubber, butadiene rubber etc. The composition ofthe elastomeric polymer material can be 5 to 50% in the compositematerial. Usually these materials are mixed with various fillers such ascarbon black, curing agents, flame retardant fillers, etc., to achievedesired properties in elastomer. For sake of simplicity, the primaryelastomer compound and all optionally included various filler materialswill be referred to generally as the elastomer compound. The elastomercompound can be used in the form of dry powder or liquid.

The coating composition will be determined by the thermal conductivity,electrical resistivity, mechanical flexibility, resiliency, and/orcompressibility requirements depending on the desired application. Inone embodiment of this invention, the coating composition includes,without limitation, about 0-90% graphite/carbon and 10-100% elastomercompound, more desirably about 0-50% graphite/carbon and 50-100%elastomer compound, and preferably about 0-30% graphite/carbon and70-100% elastomer compound. The graphite/carbon and elastomercompound(s) used in the coating composition are preferably the sametype(s) used in the PCM composite composition to which this coating willbe applied. Depending on the coating solution viscosity, the coating canbe either applied by dip coating or spray coating the PCM composite, orother widely used coating deposition techniques.

In other embodiments of this invention, the polymer coating is appliedas a polymer film, applied or laminated to the carbon matrix. The filmcan be a polymer adhesive film that is thermally bonded or pressed on atleast one surface of the composite matrix. Suitable adhesive polymerfilms are available commercially as a pure polymer form or filled withthermal conductive fillers to enhance thermal conductivity.

FIG. 3 shows a schematic of one method to manufacturing a PCM compositematerial having a polymer coating. There are many methods ofmanufacturing a PCM composite material and anyone skilled in art ofcomposite manufacture is well aware of various manufacturing techniques.Compression and injection molding are very commonly used manufacturingtechniques in the rubber and polymer industry.

Various PCM composites were manufactured successfully at high PCMloadings greater than 30% by weight by coating the composites. In oneembodiment of this invention, as shown in FIG. 3, there are basicallythree steps in the manufacture of a coated PCM composite. Referring toFIG. 3, in step one, the PCM material is mixed with graphite in powderform. Although liquid PCM can also be used, using dry powder mixingmakes the process easy to control with less complication. The PCM andgraphite powder blend is ground to fine particle size, preferably <1 mmand mixed together very well. The PCM composite is then transferred intoa mold of suitable dimensions. The molding can be done at elevatedtemperature and pressure to get a uniform mixture and also to melt thepolymer or elastomer if solid elastomer is used. The final PCM compositecan be partially or fully cured before the polymer coating is applied.Preferably, it should be partially cured so that complete curing occursafter polymer coating has been applied to form strong adhesion betweenthe coating and PCM composite structure.

Step two includes the manufacture of the polymer coating. In this step,graphite powder, if or as needed, is mixed with elastomer compoundcontaining any optional various additives (either powder or liquidform). After thorough mixing, the coating formulation is poured into abath for dip coating or the mold can be coated with coating formulationbefore transferring the PCM composite into the mold. The mixing can beperformed at below or above the glass transition temperature of thepolymer or elastomer used.

In step three, the PCM composite is molded into final form after it hasbeen coated in the previous step with the polymer coating. The moldingis usually done at high temperature and compression pressure, except fora few polymers like silicone that can also be molded at roomtemperature.

In one embodiment, the invention includes PCM impregnated into the poresof a matrix formed from a carbon/graphite fiber cloth which is flexibleand compressible. The matrix comprises phase change material disposed invoids between the fibers or bundles of the fibers, and wherein thematrix can be wrapped around one or more battery cells.

Embodiments of this invention further include a polymer coating appliedto the carbon/graphite fabric PCM composite. This polymer coatingenhances the mechanical properties of the carbon/graphite fabric PCMcomposite by increasing flexibility, compressibility of final composite,imparting mechanical resiliency, increasing electrical resistivity toprevent electrical short between cells in a battery pack, and/orreducing or eliminating PCM leakage, particularly ifnon-microencapsulated PCM is used.

FIG. 4A shows an image of a woven carbon cloth, and FIGS. 4B and 4C showthe microscopic structure of knitted and woven carbon cloths,respectively. The microscopic structure shows the individual fibers ofcarbon cloth drawn together to create a very strong mechanicalstructure. In embodiments of this invention, the voids presentin-between the individual fibers and between the fiber bundles arefilled with PCM. The surface area of carbon cloth can be very high,ranging from 10 m²/gm to 2000 m²/gm, depending on the quality and thestructure of carbon cloth from different manufacturers. Thus, it has abig potential to fill huge amount of PCM per unit weight or volume ofcarbon cloth.

The PCM material of this invention can include various and alternativeforms and combinations of phase change materials, such as describedabove. Depending on the carbon cloth density, the PCM weight fractionvaries. In one embodiment of this invention, the PCM carbon cloth ofthis invention includes about 30 to about 95% PCM and about 5 to about70% carbon/graphite cloth, and more desirably about 50 to about 95% PCMand about 5 to about 50% carbon/graphite cloth.

In one embodiment of this invention, it is preferable to add at leastone polymer material, such as described above, coating the cloth withthe applied PCM and/or to soak into the carbon cloth. The addition ofthe polymer material provides an additional binding of PCM to carboncloth fibers and also improves adhesion. The coating composition will bedetermined by the electrical resistivity, mechanical flexibility,resiliency, and compressibility requirements, depending on the desiredapplication. In one embodiment, the coating composition includes about 0to 90% graphite/carbon and about 10 to 100% elastomer compound. Asdescribed above, depending on the coating solution viscosity, thecoating can be either applied by dip coating the PCM composite or spraycoating or other widely used coating deposition techniques.

The present invention is described in further detail in connection withthe following examples which illustrate or simulate various aspectsinvolved in the practice of the invention. It is to be understood thatall changes that come within the spirit of the invention are desired tobe protected and thus the invention is not to be construed as limited bythese examples.

EXAMPLES

PCM Composite with Coating

A PCM composite with an elastomer coating was manufactured according toabove explained manufacturing process. The composition of the PCMcomposite and the elastomer coating was as follows.

PCM Composite Composition:

PCM Type: paraffin wax with melting point of 55° C.

Graphite Type: Superior Graphite™ powder

Polymer Type: Dow Corning™ Sylgard 184® elastomer—PDMS (Polydimethylsiloxane)

PCM Wt %=55%

Graphite Wt %=20%

PDMS Wt %=25%

PDMS: curing agent=10:1

Coating Composition:

Graphite weight %=40%

PDMS weight %=60%

Manufacturing Steps:

55 g of PCM powder and 20 g of graphite were mixed together in acommercial home mixer until a uniform blend was achieved. 25 g of PDMSsilicone resin and curing agent in ratio of 10:1 were mixed togetherusing a spatula and were also put inside a vacuum oven to removeair-bubbles. The PCM and graphite blend were poured into the PDMS resinliquid and they were hand mixed using a spatula. The coating formulationwas prepared by mixing 40 g of graphite powder and 60 g of PDMS siliconresin using a spatula to create a uniform blend. A rectangular mold wasobtained and the inside of the mold cavity was coated with(graphite+PDMS) coating formulation as a thin layer. The PCM compositeblend was poured into the rectangular mold and pressed to fill the moldcavity completely. The coating formulation was applied on top of the PCMcomposite in the mold cavity. The thickness of coating layer wascontrolled to obtain sufficient flexibility, resilience, andcompressibility in the coating layer. The mold was then cured at hightemperature of about 160° F. for 2-3 hours (the higher the temperature,the faster the curing of the elastomer). After curing, the final PCMcomposite was removed from the mold. The final PCM composite weight wasmeasured to calculate the density.

FIG. 5 shows the resulting PCM composite with the coating. The coatingis clearly visible in FIG. 5. In FIG. 6, a mechanical stress was appliedto the same elastomer coated PCM composite by simply bending the sampleand the sample was allowed to relax to a planar state after removing thestress. The evolution of relaxation is shown in FIG. 6. After fewseconds, the PCM composite completely recovered and returned to itsoriginal form. This property is very important in some applications,such as Li-ion battery pouch cells, where the cells constantly expandand contract during charge and discharge cycles. The PCM composite willthus be able to make thermal contact with Li-ion pouch cellcontinuously.

FIG. 7 shows various compositions of PCM composite samples that wereproduced during experimental trials. Optimal composition balancing heatstorage, thermal conductivity, and mechanical properties is highlightedin the dashed circle. Different battery applications have differentmaterial requirements and appropriate composition can be developed byvarying the ratios of the PCM, graphite or polymer to meet desiredthermal conductivity, electrical resistivity, flexibility, resilience,compressibility. Various coating compositions used in the experimentsand are identified in the FIG. 7 by the dotted line box along thetriangle edge.

The enthalpy of melting (latent heat) of a control sample withoutelastomer and a PCM composite with elastomer was measured usingdifferential scanning calorimetry (DSC). FIG. 8 shows DSC curves forpure wax and the polymer PCM composite containing various weight % ofPCM in the composite. Depending on the amount of PCM, the latent heat ofthe PCM composite material reduces proportionately. Also, the new PCMpolymer composite is very similar in shape compared to pure waxindicating no chemical reaction between the polymer (PDMS elastomer) andthe PCM.

Table 1 shows the electrical resistivity of polymer coated PCM compositematerial. The electrical resistivity varies as a function of coatingweight % and thickness. Without the polymer, the electrical resistivityof PCM and graphite matrix alone is about 0.1 to 0.4E−03 Ω·m which isabout 5-6 orders of magnitude lower than the coated PCM composite. Thus,the electrical resistivity is greatly increased, which can preventelectrical short circuits between cells. If only a PCM/graphite materialis used, an additional electrical insulation layer needs to be placedbetween the battery cell and PCM/graphite material to prevent electricalshorts between battery cells.

TABLE 1 Electrical Resistivity of Polymer coated PCM compositeElectrical Approximate Coated PCM Resistivity Coating Wt % of CoatingComposite (Ω · m) PCM Composite Thickness 1 1.39E+02 5-20% Less 24.26E+02 5-20% 3 5.89E+02 5-20% 4 6.39E+02 5-20% MoreMechanical Compression

Table 2 and FIG. 9 summarize mechanical compression testing of a controlPCM composite (graphite matrix: 20%, PCM: 80%) and a flexible PCMcomposites made according to this invention. A commercial compressionpad is also shown for comparison purpose. The compression testingincluded compression at a strain rate of 2 inches/min. The resultingforce was measured and stress was calculated from the sample area. Verystiff materials required very high force to cause a deflection (strain)whereas very resilient, soft materials required less force to cause adeflection (strain).

Table 2 lists the samples created for comparative testing of mechanicalcompression, along with measured results. The PCM composite withoutpolymer (sample 1) and including a graphite matrix (20%) required about200 psi to cause a 1% strain in the material where a commercial padrequired only 2 psi for a 25% strain (compression deflection). Thecompressible PCM composite materials according to this invention(samples 3-6) showed a stiffness that was in between the compression padand original PCM composite. With further optimization, the correctbalance of PCM, graphite, and polymer can be achieved to meet the needsof thermal and mechanical requirements that will provide the latent heatrequirements and compression requirements needed in a pouch cell orapplications requiring flexible thermal management component.

TABLE 2 Compression No. Sample Core Composition Coating CompositionForce/Strain 1 PCM Composite - PCM: 78% None 1% strain @ Non flexibleExpanded Graphite: 22% 200 psi (PCM: 80%, Graphite: 20%) 2 CommercialSilicone Foam None 25% Strain @ Compression Pad 2 psi 3 New PCM PCM: 70%Graphite powder: 40% 5% Strain @ Composite with Graphite powder: 15%,Silicone Polymer: 60% 10 psi polymer coating Silicone Polymer: 15% 4 NewPCM PCM: 70% Graphite powder: 40% 5% Strain @ Composite with Graphitepowder: 15%, Silicone Polymer: 60% 10 psi polymer coating SiliconePolymer: 15% 5 New Carbon Cloth PCM: 60% None 10% Strain @ PCMComposite - Carbon Cloth: 40% 11 psi without Polymer coating 6 NewCarbon Cloth PCM: 60% Graphite powder: 40% 10% Strain @ PCM Composite -Carbon Cloth: 40% Silicone Polymer: 60% 10 psi with Polymer coatingCarbon Cloth-PCM Composite

A small piece of carbon cloth was cut out and soaked in liquid paraffinwax PCM with a melting temperature of 55° C. The soaking was conductedat 75° C. and −75 kPa in a vacuum oven for one hour.

Experimental Details:

Carbon cloth dimensions: 10 cm×10 cm

PCM soaking conditions: soaked at 75° C., −75 kPa vacuum for 1 hour

Weight before soaking=3.68 gm

Weight after soaking=8.86 gm

Weight % of PCM=58.5%

The above sample was cut into two halves and one piece was coated with apolymer/graphite mixture to prevent any PCM leakage and also to increaseelectrical resistivity to prevent electrical short when used with Li-ioncell. The coating composition is listed below. The coating was appliedusing a spatula on all sides of carbon cloth containing PCM.

Coating Composition:

Graphite powder-Superior Graphite=4 gms

PDMS solvent+curing agent (Dow Corning Sylgard 184)=6 gms

Thermal Soaking Test:

The carbon cloth samples with and without polymer coating were putinside a thermal oven maintained at 65° C. The melting temperature ofPCM inside the carbon cloth was 55° C. and thus the PCM was meltedcompletely and if the pores of carbon cloth were big enough, the PCMshould leak out. If the polymer coating is effective, the PCM weightloss should be very minimal or negligible depending on the coatingthickness.

The carbon cloth initially had about 58.5 wt % of PCM and after exposureat high temperature (>melting temperature of PCM), the weight loss wasabout 13.5% on carbon cloth without coating. The PCM weight lossremained rather constant after some time, as shown in the resultssummary of FIG. 10. This suggests that the PCM held in large pores ofcarbon cloth were easily lost whereas the PCM held inside verymicroscopic pores or void spaces in between carbon fibers were heldtightly. On the other hand, the carbon cloth coated with apolymer/graphite mix showed significant reduction in PCM weight loss inthe liquid state. Thus, the polymer coating of this invention waseffective in preventing PCM weight loss in liquid state and acted as abarrier for PCM diffusion. By proper optimization of coating compositionand coating thickness, the PCM weight loss can be completely prevented.Another advantage was that the coating also increases the electricalresistivity of the final carbon cloth composite. This can reduce orprevent electrical shorts in between the battery cells when used inbattery thermal management applications. Without the coating, additionaldielectric components have to be assembled in between battery cells andPCM composite to prevent electrical short, because the PCM/graphitecomposite has a low electrical resistivity.

The enthalpy of melting (latent heat) of the carbon cloth/PCM compositewas compared with an expanded graphite/PCM composite. The measurementwas performed using DSC. FIG. 11 shows the DSC curves for baselinesamples: the expanded graphite/PCM composite and the carbon cloth/PCMcomposite. The PCM weight % in the baseline and carbon cloth compositewas 75% and 55%, respectively. Depending on the amount of PCM availablein the composite, the latent heat of the PCM composite material reducedproportionately.

High Surface Area Carbon Cloth

Depending on the surface area of the carbon cloth, PCM impregnation canbe significantly improved.

PCM Composite:

Carbon Cloth Dimensions: 10 cm×25 cm

Soaking Conditions: 160° F. under vacuum at −50 kPa for 2 hrs

PCM Wt %=75-82%

Carbon Cloth Wt %=18-25%

Coating:

Polymer (PDMS™ Silicone from Dow Corning)=90%

Polymer Curing Agent=10%

A thick polymer coating of about 3 mm, which is similar to a typicalcompression pad thickness, was applied on the PCM impregnated carboncloth. The polymer was mixed with curing agent in 10:1 ratio and pouredinto a small mold. The PCM impregnated carbon cloth was then immersed inthe polymer and more polymer was applied to completely cover the carboncloth composite. The sample was then cured at 160° F. in an ovenovernight for >10 hrs. Exemplary samples are shown in FIG. 12, and thelatent heat was 145 J/g which is very comparable to the PCM compositewithout the polymer coating. The outer polymer coating was also soft andresilient, and was very close to meeting the specifications of acommercial compression pad. Further refinements can be made by choice ofpolymer and/or weight compositions. This example demonstrates that atruly multifunctional PCM composite can be developed that has goodthermal energy storage, thermal conductivity, and the mechanicalrobustness needed to meet battery applications such as pouch cells orthermal energy storage application.

Thus, the invention provides carbon/PCM composite materials withincreased compressibility and flexibility, and that can withstandmechanical stresses without breaking apart or losing thermal contactwith cells. The carbon cloth/PCM composites of this invention do notneed any polymer or elastomer to impart mechanical strength,flexibility, compressibility, thus eliminating the need for additionalcomponents, reducing cost, and simplifying manufacturing. The polymercoating prevents PCM leakage, thus reducing or eliminating the need forusing microencapsulated PCM and thereby reducing cost. The polymercoating also increases electrical resistivity, thereby reducing oreliminating a need for additional insulating components to be includedin the PCM/graphite composite.

The invention illustratively disclosed herein suitably may be practicedin the absence of any element, part, step, component, or ingredientwhich is not specifically disclosed herein.

While in the foregoing detailed description this invention has beendescribed in relation to certain preferred embodiments thereof, and manydetails have been set forth for purposes of illustration, it will beapparent to those skilled in the art that the invention is susceptibleto additional embodiments and that certain of the details describedherein can be varied considerably without departing from the basicprinciples of the invention.

What is claimed is:
 1. A thermal management composite, comprising: acarbon or graphite matrix including a phase change material; and anelastomeric binder material impregnated throughout the matrix; and anelastomeric polymer coating on at least one outer surface of thecomposite.
 2. The composite of claim 1, wherein the matrix compriseswoven or nonwoven carbon or graphite fibers.
 3. The composite of claim2, wherein the matrix comprises phase change material disposed in voidsbetween the fibers or bundles of the fibers, and a polymer coating on atleast one surface of the matrix, and wherein the matrix can be wrappedaround one or more battery cells.
 4. The composite of claim 3, whereinthe matrix comprises a molded composite.
 5. The composite of claim 4,wherein the phase change material comprises a microencapsulated wax. 6.The composite of claim 4, further comprising a polymer coating on eachof a top side and a bottom side of the matrix.
 7. The composite of claim1, wherein the matrix comprises a molded composite.
 8. The composite ofclaim 7, wherein the polymer coating comprises graphite dispersedtherein.
 9. The composite of claim 1, wherein the phase change materialcomprises a wax.
 10. The composite of claim 1, further comprising amicroencapsulated phase change material.
 11. The composite of claim 1,carbon or graphite matrix comprises a molded carbon or graphite powder.12. A method of making a thermal management composite, the methodcomprising: impregnating a carbon or graphite matrix with a phase changematerial and an elastomer binder material; and coating at least one sideof the impregnated matrix with an elastomeric polymer coating.
 13. Themethod of claim 12, wherein the matrix comprises woven or nonwovencarbon or graphite fibers.
 14. The method of claim 12, furthercomprising: mixing the phase change material and the elastomer bindermaterial with a carbon or graphite material to obtain a matrix mixture;molding the matrix mixture into the impregnated matrix of the carbon orgraphite matrix impregnated with the phase change material and theelastomer binder material; and coating the at least one side of theimpregnated matrix with the elastomeric polymer coating.
 15. The methodof claim 12, further comprising dip coating or spray coating the matrixwith the phase change material and/or the polymer coating.
 16. Themethod of claim 12, wherein the coating comprises a coating polymermaterial mixed with carbon or graphite powder or fibers.
 17. The methodof claim 14, further comprising dip coating or spray coating the matrixwith the phase change material and/or the polymer coating.
 18. A thermalmanagement composite, comprising: a matrix of carbon or graphite fibersor bundles of the fibers with a plurality of voids between the fibers orthe bundles of the fibers within the matrix, the matrix having an outersurface; a phase change material dispersed throughout the matrix withinthe voids; an elastomeric binder material dispersed throughout thematrix within the voids; and an elastomeric polymer coating on the outersurface of the composite.